Dumb one, but (CC source)

[KC2IXE]

Ok, dumb one (it's been YEARS since I was doing electronics, and just want to check before I 'make smoke')

Assuming I have a LED rated for 20mA, and I want to drive this with a dead simple Constant current source, would this work (doing the math/design right)

LM117/317

62 ohm (nominal) resisitor between Vout and Adj - hang the LED between Vout and ground - drive the circuit with say 10-13.8 volts (car battery - nominal could even use a 9v battery)
And yes, I know about hanging an input cap on Vin

Sound right (yes, I know we have to go to standard resistor values too etc)

 

[Mr Happy]

For 20 mA it isn't worth using a complicated regulator. Just put a resistor in series with the LED. A high voltage dropped over a resistor is the simplest constant current source you can design.

 

[KC2IXE]

For 20 mA it isn't worth using a complicated regulator. Just put a resistor in series with the LED. A high voltage dropped over a resistor is the simplest constant current source you can design.

Problem is voltage will be all over the place - from a low of say 6, to a high of 14

 

[legtu]

Ok, dumb one (it's been YEARS since I was doing electronics, and just want to check before I 'make smoke')

Assuming I have a LED rated for 20mA, and I want to drive this with a dead simple Constant current source, would this work (doing the math/design right)

LM117/317

...

Yes, it should work.

 

[jtr1962]

Try this circuit instead:

This circuit is very versatile. It's a linear circuit so you'll need to heat sink Q2 if you're driving power LEDs, but should be fine without heatsinking for driving a 20 mA LED. Also, you'll need to use a power transistor (something like a TIP31) for Q2 instead of the small signal PN2222A shown on the schematic for driving power LEDs. However, for small 20 mA LEDs it should work just fine as shown. As shown the circuit as shown will drive the LED(s) at 20 mA. Even though the schematic has 3 LEDs, it will drive one just fine, and will provide 20 mA to the LED if voltage varies over the 6 to 14 volt range you stated.